The present invention relates to neutron detectors which are used to detect neutrons, and more particularly to a new improved solid state detector that is sensitive to both thermal (slow) and fast neutrons.
Current thermal neutron detectors include devices which operate as ionization chambers or proportional counters, both of which use a neutron-active gas such as B.sup.10 F.sub.3 or He.sup.3. These materials have a large absorption cross section for thermal neutrons and, upon absorption of a neutron, release energetic reaction particles. These particles produce ionizations in the surrounding gas which are detected by appropriately biased electrodes. Other detectors coat the walls of the ionization chamber with a solid neutron-active material such as Li.sup.10, B.sup.10, or U.sup.235. These materials absorb neutrons and similarly release particles which produce ionizations.
Thermal neutron detectors can be used to detect fast neutrons by surrounding the detector with a moderating hydrogenous material. The moderating material continuously slows the fast neutrons and the detector detects the thermalized neutrons. Various unfolding codes and other techniques have been developed to provide a rough estimate of the original neutron energy distribution.
Ionization chambers are also used in fast neutron detector designs. The detector is designed so that fast neutrons collide with hydrogen thereby creating energetic free protons which produce ionizations detectable by appropriately biased electrodes.
Solid state neutron detectors detect electron-hole pairs that cross a semiconductor junction, the electron-hole pairs being produced by reaction particles formed as a result of neutron absorptions within films or dopants of neutron active material incorporated within the detector. Solid state detectors generally use silicon, germanium, or silicon carbide as a semiconducting material.
Ross, U.S. Pat. No. 3,227,876, describes a silicon solid state semiconductor detector with a p-n junction wherein one of the layers is doped with B.sup.10, a neutron sensitive material. Incident slow neutrons are absorbed by the boron thereby creating energetic reaction particles which, in turn, create electron-hole pairs that diffuse into and across the junction to produce a current pulse. Reaction particles that backscatter away from the junction often are not detected since these particles either exit the detector without creating a significant number of electron-hole pairs or the electron-hole pairs formed by these particles are too far removed from the junction to migrate across it. To detect fast neutrons, this detector is surrounded by a moderator that slows the neutrons thereby creating thermal neutrons which are then detected. This detector has a low sensitivity per unit volume since the doping of the detector limits the amount of neutron-sensitive material it contains. It also cannot accurately determine the incident neutron energy distribution because of the inherent inaccuracies of present unfolding codes which are required since only moderated thermal neutrons are detected.
Finally, a diamond crystal plate solid-state detector has been used to detect both fast and thermal neutrons. Kozlov, U.S. Pat. No. 3,805,078, describes such a diamond crystal detector. A diamond crystal is expensive, thereby precluding detector designs covering large surface areas, and the diamond crystal tends to polarize since a portion of the electrons and holes migrating through it become trapped.
Other prior art solid state detectors are described in Butler, U.S. Pat. No. 4,000,502; Chung, M. K., "Ge(Li) Surface barrier Detectors for Fast Neutron Spectroscopy", Journal of the Korean Physical Society, Vol. 8, No. 1, (March 1975); and Dearnaley, G., "Semiconductor Fast Neutron Detectors", I.R.E. Transactions on Nuclear Science, Vol. NS-9, No. 3, (June 1962).